The invention relates to a holding apparatus for the transport of conveyed items, in particular the lifting, transporting, imbricating and stacking of ferromagnetic conveyed parts, having at least one permanent magnet for generating a permanent magnetic field, also at least one electrical magnet coil for generating a temporary magnetic field and compensation of the permanent magnetic field, and having a magnetically conductive housing for the permanent magnet and the magnet coil, the permanent magnet usually being magnetized in the direction of its smallest dimension, the permanent magnet also being arranged in the direction of magnetization at the rear with one pole against the housing, forming a magnetic yoke, and at the front with the other pole in the direction of a working clearance opposite the conveyed parts, and the housing having in cross section two or more webs rising up from a baseplate. For the purposes of the invention, a working clearance is generally to be understood as a clearance of which the width is dimensioned such that a displacement or compensation of the permanent magnetic field by the temporary magnetic field takes place principally at this location. That is to say, all the magnetic field changes to be set for the secure holding and letting go of the conveyed parts are ideally restricted to the region of the working clearance. The webs are of a regular finger-like design in cross section.
A holding apparatus of the type described at the beginning is disclosed by German Patent 34 23 482.
In this case there is provided a single permanent magnet, which is arranged centrally in a magnetically conductive iron yoke. Arranged between this permanent magnet and the working clearance are a magnetically conductive compensation plate with a magnetically conductive web and a magnetically conductive central pole, in this sequence. This is disadvantageous with regard to possible stray fields. This is so since the permanent magnet is not arranged directly in the region of the working clearance but is set back from it. As a consequence, losses in magnetic field strength are unavoidable.
Added to this is that only one electrical coil is provided between the compensation plate and the central pole for the compensation or displacement of the magnetic field out of the region of the working clearance. That is to say, this one coil must be designed such that the permanent magnetic field is reliably compensated. Adaptation to differently designed conveyed parts is not accomplished by this means. It must also be taken into consideration that, due to the necessary design of the electrical coil to suit the maximally occurring adhesive forces between the conveyed part and the holding apparatus, the achievable switching frequency is often adversely influenced. This can be attributed to the fact that coils of adequate magnetic field strength for generating an opposing field have at the same time a great inductance (measured in henries).
According to Lenz's law, however, large inductances (formerly self-inductance coefficients) effect a lessening or slowing of the rise of a switching current, with the result that the switching frequency is reduced. This is to be regarded as disadvantageous in particular in the transport of conveyed items with the required high cycle times.
Austrian Patent 282 878 discloses a load lifting magnet for handling ferromagnetic loads, having a plurality of magnetic poles and excitation windings assigned to the poles, the load lifting magnet having a primary magnetic circuit which has an internal group of first poles and at least one first excitation winding which can be switched on independently. There is also provided an additional magnetic pole, which is magnetically isolated from the primary circuit and to which there is assigned at least one second excitation winding, which can be switched on independently. This is intended overall to provide a load lifting magnet with which it is possible to be certain to lift only the uppermost plate in each case off a stack of single ferromagnetic plates. At the same time, it is intended to be ensured that, during further transport of the lifted-off plate, an adequately great adhesive force can be exerted on the single plate. If need be, it is also to be possible to transport safely relatively large customary ferromagnetic loads. --Problems associated with the achievable switching frequencies are not mentioned. --This is where the invention overall wishes to provide a remedy.
The invention is based on the object of developing a holding apparatus of the embodiment described at the beginning in such a way as to make it possible to adapt to different conveyed parts while retaining at the same time a high switching frequency and simple construction.